In the use of prior art scanning and digital imaging systems for reflective scanning, as for example, flat-bed scanner systems, an original on an opaque substrate is placed with the surface containing the original facing down on a flat transparent reference surface, typically glass. The original document is fixed on the surface such that a line of the original, herein after referred to as a "scan line" is illuminated from below, and the light reflected from the scan line is directed through an optical system to form an image of the scan line on a sensor, such as a CCD array. The sensor, or CCD, converts the optical signal to an electronic representation of the scan line, comprising a line of digital picture elements or "pixels". Typically, the desired portion of the original is scanned by moving the illumination system, optical system and CCD sensor relative to the original along a direction hereinafter referred to as the "scanning axis" or y-axis. However, systems also exist in which the original document is moved relative to a fixed optical system.
In general, a CCD has a plurality of photosensor arrays for sensing the intensity of monochromatic, or color light (when supplied with the appropriate RGB color filter stripes). A typical trilinear color sensor, or trilinear CCD, is described in the publication entitled "An Introduction to Digital Scanning," Digital Colour Prepress volume four, Agfa Prepress Education Resources, P.O. BOX 7919, Mt. Prospect, Ill., the disclosure of which is hereby incorporated by reference. As known in the art, a typical color trilinear CCD is comprised of three rows of photosensor elements or pixels. Each row of photosensor elements, or photosensor array, is covered by a red, green or blue integral filter stripe for spectral separation. When the CCD is viewed on edge in the scanning direction, the three color pixel sensor arrays red, green and blue (RGB) are separated by a channel. Typically, the length of each channel is equivalent to an integer number of optical pixel lengths. Since the width of each RGB sensor is also an optical pixel length, the interchannel spacing between R-G and G-B sensors is equivalent to an integer number of pixel lengths. The interchannel spacing, or distance between sensors, is hereinafter referred to as the optical line spacing (OLS).
As known in the art, a pixel within a CCD sensor array is square and determines the optical resolution for a scan. For example, CCD part number KLI-8013 manufactured by the EASTMAN KODAK Company has red, green and blue sensor arrays spaced equally apart in the scanning direction by a distance of 108 .mu.m or 12 pixels/lines. Each pixel length is 9 .mu.m, and therefore, a square pixel measures 9.times.9 .mu.m.sup.2.
Typically, CCDs of the prior art have one common transfer gate receiving means so all three color sensors are activated to scan simultaneously. However, because of the optical line spacing of the CCD's photosensor elements any real time scan results in three different line scans, one for each color. At optical resolution, for example, the scan line for the blue pixels is shifted by a distance equal to the CCD optical line spacing with respect to the scan line for the green pixel data; the scan line for the green pixels is shifted by the same distance with respect to the scan line for the red pixel data and the scan line for the blue pixels is shifted by twice this distance with respect to the scan line for the red pixel data. At resolutions in which the optical line spacing divided by the scanned pixel size is an integer, there is no color misregistration. Thus, post processing and memory are all that is needed to correlate, or register, the integer shifted line scans for each color.
Heretofore, the problem which remained unresolved by color, monochromatic or digital imaging systems of the prior art related to resolution scans in which the optical line spacing divided by the scanned pixel size was not equal to an integer. Where the lineskip has a fractional component, there exists a misregistration or fractional shift in color pixel data which can not be easily registered without severely degrading overall system performance. Although it is possible to perform these other resolution scans or digital exposures with a CCD imaging system in one pass, problems of artifacts, poor image quality and color misregistration occur as a result.
In prior art imaging systems such as digital cameras and scanners where the original color scene is illuminated and converted into electronic signals via a linear CCD, typically the misregistration problem is solved by superimposing three separate scans or exposures of the original image. Each scan or exposure processes a different primary color--red, green and blue. Making multiple scans of the same original, of course, increases the amount of memory and processing needed to complete an image and accentuates problems with mechanical registration, illumination and CCD stability.
Heretofore, single pass systems of the prior art attempted to solve the fractional registration problem through extensive post processing, resampling and filtering or by simply ignoring the phase shifted data and outputting an imperfect scan. These solutions resulted in either poor quality images and/or poor system performance.
One solution for a single pass imaging system would be to increase the optical resolution of the system so that a larger sampling of data than that needed for a particular resolution is performed. The resulting "over-samplings" of pixel data could then be filtered with the extraneous data discarded. However, not only would system performance be degraded by excessively slow scan speeds but the memory requirements either on board the imaging device or elsewhere in the system would seriously degrade performance.
The present invention solves the problems of the prior art without increasing memory requirements and without extensive post processing and filtering of raw shifted pixel data. The need for time consuming multi-pass scanning or imaging is eliminated. Further, the teachings of the present invention reduce the need for CCDs with expensive precision made optical line spacings.